The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the Universal Mobile Telecommunication System (UMTS) and the fourth-generation wireless system commonly known as Long Term Evolution (LTE). The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network (E-UTRAN). LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink (the link carrying transmissions from the base station to a mobile station) and in the uplink (the link carrying transmissions from a mobile station to the base station). To support high data rates, LTE allows for a system bandwidth of 20 MHz, or up to 100 MHz when carrier aggregation is employed. LTE is also able to operate in different frequency bands and can operate in at least Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes.
In LTE, transmissions from base stations (eNBs) are sent to mobile stations (referred to as user equipment, or UEs) using orthogonal frequency division multiplexing (OFDM). OFDM splits the signal into multiple parallel sub-carriers in frequency. LTE uses OFDM in the downlink and Discrete Fourier Transform (DFT)-spread OFDM (also referred to as single-carrier Frequency Division Multiple Access, or SC-FDMA) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid, where each resource element corresponds to one OFDM subcarrier in frequency during one OFDM symbol time interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink. The basic unit of transmission in LTE is a resource block (RB), which in its most common configuration consists of 12 subcarriers in the frequency domain and 7 OFDM symbols (one slot, or 0.5 ms). A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. A unit of one subcarrier and 1 OFDM symbol is referred to as a resource element (RE). Thus, an RB consists of 84 REs.
An LTE radio subframe is composed of two slots in time and multiple resource blocks in frequency with the number of RBs determining the bandwidth of the system. Furthermore, the two RBs in a subframe that are adjacent in time are denoted as an RB pair. Currently, LTE supports standard bandwidth sizes of 6, 15, 25, 50, 75 and 100 RB pairs.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds in length, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of e.g. the control information.
The signal transmitted by an eNB in a downlink subframe may be transmitted from multiple antennas and the signal may be received at UE that has multiple antennas. In order to demodulate any transmissions on the downlink, a UE thus relies on reference symbols (RS) that are transmitted on the downlink. These reference symbols and their positions in the time-frequency grid are known to the UE and can be used to determine channel estimates by measuring the effect of the radio channel on these symbols. As of Release 11 (Rel-11) of the 3GPP specifications for LTE, there are multiple types of reference symbols. One important type is the common reference symbols (CRS), which are used for channel estimation during demodulation of control and data messages. The CRSs are also used by the UE for synchronization, i.e., to align its timing with the downlink signal as received from the eNB. The CRSs occur once every subframe.
From 3GPP LTE Release 11 (Rel-11) onwards, above described resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (ePDCCH). For Rel-8 to Rel-10 only Physical Downlink Control Channel (PDCCH) is available. The PDCCH/ePDCCH is used to carry downlink control information (DCI) such as scheduling decisions and power-control commands More specifically, the DCI includes:                Downlink scheduling assignments, including Physical Downlink Shared Channel (PDSCH) resource indication, transport format, hybrid-ARQ information, and control information related to spatial multiplexing (if applicable). A downlink scheduling assignment also includes a command for power control of the Physical Uplink Control Channel (PUCCH) used for transmission of hybrid-ARQ acknowledgements in response to downlink scheduling assignments.        Uplink scheduling grants, including Physical Uplink Shared Channel (PUSCH) resource indication, transport format, and hybrid-ARQ-related information. An uplink scheduling grant also includes a command for power control of the PUSCH.        Power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.        
One PDCCH/ePDCCH carries one DCI message containing one of the groups of information listed above. As multiple terminals can be scheduled simultaneously, and each terminal can be scheduled on both downlink and uplink simultaneously, there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on separate PDCCH/ePDCCH resources, and consequently there are typically multiple simultaneous PDCCH/ePDCCH transmissions within each subframe in each cell. Furthermore, to support different radio-channel conditions, link adaptation can be used, where the code rate of the PDCCH/ePDCCH is selected by adapting the resource usage for the PDCCH/ePDCCH, to match the radio-channel conditions.
There is a start symbol for PDSCH and ePDCCH within the subframe. The OFDM symbols in the first slot are numbered from 0 to 6. For transmissions modes 1-9, the starting OFDM symbol in the first slot of the subframe for ePDCCH can be configured by higher layer signaling and the same is used for the corresponding scheduled PDSCH. Both sets have the same ePDCCH starting symbol for these transmission modes. If not configured by higher layers, the start symbol for both PDSCH and ePDCCH is given by the Control Format Indicator (CFI) value signaled in the Physical Control Format Indicator Channel (PCFICH).
LTE Measurements
A UE performs periodic cell search and Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) measurements in Radio Resource Control (RRC) Connected mode. It is responsible for detecting new neighbor cells, and for tracking and monitoring already detected cells. The detected cells and the associated measurement values are reported to the network. Reports to the network can be configured to be periodic or aperiodic based on a particular event.
License Assisted Access
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited which cannot meet the ever increasing demand for larger throughput from applications/services. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LTE should consider the coexistence issue with other systems such as IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is also known under its marketing brand “Wi-Fi.” Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum can seriously degrade the performance of WLAN/Wi-Fi devices as a WLAN/Wi-Fi device will not transmit once it detects the channel is occupied.
3GPP License Assisted Access (LAA) intends to allow LTE equipment to also operate in the unlicensed radio spectrum such as the 5 GHz band. The unlicensed spectrum is used as a complement to the licensed spectrum. One way to utilize the unlicensed spectrum reliably is to transmit essential control signals and channels on a licensed carrier. Accordingly, devices connect in the licensed spectrum (primary cell or PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum (secondary cell or SCell). To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell. An example is when a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. The secondary cell in unlicensed spectrum is referred to as a license assisted secondary cell (LA SCell).
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a listen-before-talk (LBT) access method needs to be applied. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 WLAN standard.
The use of the LBT procedure leads to uncertainty at the eNB regarding whether it will be able to transmit a downlink subframe(s) or not, at any given time. This leads to a corresponding uncertainty at the UE as to if it actually has a subframe to decode or not. An analogous uncertainty exists in the uplink direction where the eNB is uncertain if the UEs scheduled on the SCell actually transmitted or not.
In typical deployments of WLAN, carrier-sense multiple access with collision avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed to perform a clear channel assessment (CCA), and a transmission is initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is deemed to be Idle. When the coverage areas of several access points (APs) using the same frequency overlap, this means that all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP which is within range can be detected. Effectively, this means that if several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be severely degraded.
Before a transmission burst on the LAA SCell, the device/equipment employing LBT, performs a Clear Channel Assessment (CCA) check using “energy detect”. The device/equipment observes a Operating Channel(s) for defer period and a random number of observation slots. If the channel is found to be idle during these periods, the device can transmit for duration up to a transmission opportunity (TXOP). The purpose of the defer period is to avoid colliding with WLAN acknowledgement (ACK) frame transmissions (without LBT) following a WLAN data frame burst. The random number of idle observation slots is designed to randomize the start of transmissions from different nodes that want to access the channel at the same time.
Standalone operation in unlicensed spectrum using LTE
In addition to LAA operation, it should be possible to run LTE fully on the unlicensed band without the support from the licensed band. This is called LTE-Unlicensed (LTE-U) Stand Alone. An industry alliance (MuLTEfire Alliance or MFA) develops specifications for Standalone LTE-U operations to operate LTE in unlicensed spectrum without the aid of a licensed carrier. In Standalone operations, the PCell will also operate on the unlicensed carrier and thus essential control signals and channels will also be potentially subject to interference and LBT. New aspects compared to licensed spectrum are for example: PCell is on an unlicensed band and the UE is required to initiate, establish, maintain connection with the PCell; mobility management must work in an unsynchronized unplanned network; and mobility manangement must work in an environment with dynamic neighbor relations. Further the carrier (re)selection process (when the network node changes its carrier frequency during operation) becomes more problematic when it is also applied to the PCell (or serving cell in IDLE mode), because then there is no cell that the UE is “anchored” to during the carrier frequency change.
LAA Discovery Reference Signal (DRS)
The discovery reference signals or discovery signals in a DRS occasion comprise the primary synchronization signal (PSS), secondary synchronization signal (SSS), CRS and when configured, also the channel state information reference signals (CSI-RS). The PSS and SSS are used for coarse synchronization, when needed, and for cell identification. The CRS is used for fine time and frequency estimation and tracking and may also be used for cell validation, i.e., to confirm the cell identity (ID) detected from the PSS and SSS. The CSI-RS is another signal that can be used in dense deployments for cell or transmission point identification.
The DRS occasion corresponding to transmissions from a particular cell has a duration of one subframe with the last two symbols removed
The discovery signals should be useable by the UE for performing cell identification, RSRP and RSRQ measurements. The RSRP measurement definition based on discovery signals is the same as in prior releases of LTE. The Received Signal Strength Indicator (RSSI) measurement is defined as an average over all OFDM symbols in the downlink parts of the measured subframes within a DRS occasion.
When a UE is being served on multiple carrier frequencies via a PCell and one or more SCells, the UE needs to perform radio resource management (RRM) measurements on other cells on the currently used carrier frequencies (intra-frequency measurements) as well as on cells on other carrier frequencies (inter-frequency measurements). Since the discovery signals are not transmitted continuously, the UE needs to be informed about the timing of the discovery signals so as to manage its search complexity. Furthermore, when a UE is being served on as many carrier frequencies as it is capable of supporting and inter-frequency RRM measurements need to be performed on a different carrier frequency that is not currently being used, the UE is assigned a measurement gap pattern. This gap pattern on a serving frequency allows the UE to retune its receiver for that frequency to the other frequency on which measurements are being performed. During this gap duration, the UE cannot be scheduled by the eNB on the current serving frequency. Knowledge of the timing of the discovery signals is especially important when the use of such measurement gaps is needed. Beyond mitigating UE complexity, this also ensures that the UE is not unavailable for scheduling for prolonged periods of time on the current serving frequencies (PCell or SCell).
The provision of such timing information is done via a discovery measurement timing configuration (DMTC) that is signaled to the UE. The DMTC provides a window with a duration of 6 ms occurring with a certain periodicity and timing within which the UE may expect to receive discovery signals. The duration of 6 ms is the same as the measurement gap duration as defined currently in 3GPP LTE and allows the measurement procedures at the UE for discovery signals to be harmonized regardless of the need for measurement gaps. Only one DMTC is provided per carrier frequency including the current serving frequencies. The UE can expect that the network will transmit discovery signals so that all cells that are intended to be discoverable on a carrier frequency transmit discovery signals within the DMTCs. Furthermore, when measurement gaps are needed, it is expected that the network will ensure sufficient overlap between the configured DMTCs and measurement gaps.
Radio Link Monitoring (RLM)
When a UE is connected to a wireless network, if radio link conditions between the UE and its serving cell deteriorate beyond a certain point then the UE determines that a Radio Link Failure (RLF) has occurred. This could occur in a situation when the UE enters a fading dip, for example, or if a handover was needed but the handover fails for one reason or another. RLF triggers certain actions by the UE, such as the initiation of a radio resource control (RRC) re-establishment procedure. The UE is configured by the network (e.g., via RRC signaling) with one or more parameters that control the RLF triggering conditions for the UE. For LTE systems, these are described in section 5.3.11 of 3GPP Technical specification (TS) 36.331.
The quality of the radio link is monitored in the UE, on the physical layer (Layer 1), as described in the most recent versions of 3GPP TS 36.300, 3GPP TS 36.331, and 3GPP TS 36.133, and as summarized below. Note that in this disclosure, “layer” refers to a protocol layer as implemented by a processing circuit executing appropriate firmware and/or software. Thus, a typical UE may comprise one or more processing circuits executing a protocol stack, such that the UE may be regarded as comprising several “layers,” including the physical layer (Layer 1, L1), a data link layer (Layer 2, L2), a network layer (Layer 3, L3), etc.
When the UE is in RRC_CONNECTED state and upon detecting that the physical layer is experiencing reception problems with respect to receiving signals from the primary cell (PCell), e.g., according to criteria defined in 3GPP TS 36.133, the physical layer sends, to the RRC protocol layer, an indication of the detected problems. This indication is referred to as an “out-of-sync” indication. After a configurable number N310 of such consecutive out-of-sync indications, a timer T310 is started. If the physical layer subsequently generates N311 consecutive “in-sync” indications for the PCell while T310 is running, then the RRC layer will stop timer T310. On the other hand, if the link quality is not improved (recovered) while timer T310 is running, i.e., if there are not N311 consecutive “in-sync” indications from the physical layer, a RLF is declared in the user equipment when the timer T310 expires. This sequence of events is shown in FIG. 1. The functions of the currently relevant timers and counters described above are listed in Table 1, for reference.
TABLE 1TimerStartStopAt expiryT310Upon detectingUpon receiving N311If security is notphysical layerconsecutive in-syncactivated: go toproblems, i.e. uponindications from lower layers,RRC_IDLEreceiving N310upon triggering the handoverelse: initiate theconsecutive out-of-procedure and upon initiatingconnection re-sync indications fromthe connection re-establishmentlower layersestablishment procedureprocedureT311Upon initiating theSelection of a suitable E-Enter RRC_IDLERRC connection re-UTRA cell or a cell usingestablishmentanother RATprocedureConstantUsageN310Maximum number of consecutive “out-of-sync” indications received fromlower layersN311Maximum number of consecutive “in-sync” indications received fromlower layers
The UE may read the timer values and counter constants shown in Table 1 from system information broadcasted in the cell. Alternatively, it is possible to configure the UE with UE-specific values of the timers and counter constants using dedicated signaling, i.e., where specific values and constants are given to a particular UE or group of UEs with messages directed only to that UE or group of UEs.
If timer T310 expires, indicating that an RLF has occurred, then the UE initiates a connection re-establishment to recover the ongoing RRC connection. This procedure includes cell selection by the user equipment. That is, the RRC_CONNECTED UE shall autonomously try to find a better cell to connect to, since the connection to the previous cell failed according to the described measurements. It could occur that the UE returns to the first cell anyway, but the same procedure is executed in any event. Once a suitable cell is selected as further described, e.g., in 3GPP TS 36.304, the UE requests a re-establishment of the connection in the selected cell. It is important to note the difference in mobility behavior when an RLF results in UE-based cell selection, in contrast to the normally applied network-controlled mobility.
If the re-establishment is successful, which depends on, among other things, whether the eNB controlling the selected cell is prepared to maintain the connection to the UE, which implies that it is prepared to accept the re-establishment request, then the connection between the UE and the network can resume, through the newly selected eNB (or the re-selected eNB, if the connection is re-established to the same eNB). In LTE, a re-establishment procedure includes a random-access request in the selected cell, followed by higher layer signaling where the user equipment sends a message with content that be used to identify and authenticate the UE. This is needed so that the network can trust that it knows exactly which UE is attempting to perform the re-establishment.
If the re-establishment attempt fails, the UE goes to RRC_IDLE state and the connection is released. To continue communication, a new RRC connection must then be requested and established. A re-establishment failure could occur, for example, if the eNB that receives the re-establishment request is unable to identify the UE that requests the re-establishment. Such a condition may occur if the receiving eNB has not been informed or otherwise prepared for a possible re-establishment from this UE.
The reason for introducing the timers T31x and counters N31x described above is to add some freedom and hysteresis for configuring the criteria for when a radio link should be considered as failed and needing to be re-established. This flexibility is desirable, since it would affect the end-user performance negatively if a connection is abandoned prematurely if it turned out that the loss of link quality was temporary and the UE succeeded in recovering the connection without any further actions or procedures, e.g., before T310 expires or before the counter reaches value N310.
In LTE networks, the radio link is monitored by the UE using Qin and Qout status, as specified by the 3GPP. Details may be found in chapter 4.2.1 of the 3GPP document 3GPP TS 36.213, v. 10.10.0 (June 2013) and chapter 7.6 of the 3GPP document 3GPP TS 36.133, v10.12.0 (September 2013), both of which documents are available at 3gpp.org.
The UE estimates the downlink radio link quality and compares it to the thresholds Qout and Qin for the purpose of monitoring downlink radio link quality of the primary cell (PCell) and determining “in-sync” and “out-of-sync” conditions. The threshold Qout is defined as the level at which the downlink radio link cannot be reliably received, and corresponds to a 10% block-error rate of a hypothetical PDCCH transmission, taking into account errors on the PCFICH. The threshold Qin is defined as the level at which the downlink radio link quality can be significantly more reliably received than at Qout, and corresponds to a 2% block-error rate of a hypothetical PDCCH transmission, taking into account PCFICH errors. Thus, the UE is considered “in-sync” when the monitored radio link quality is better than Qin, and is “out-of-sync” when the monitored radio link quality is worse than Qout.
Rel-13 LTE RLM is time-based and counter-based. In non-discontinuous reception (non-DRX), L1 out-of-sync and in-sync evaluation period is based on 200 ms and 100 ms of downlink subframes respectively. In DRX, Layer 1 (L1) out-of-sync and in-sync evaluation period is based on a number of DRX cycles. Layer 3 (L3) counters are based on receiving consecutive L1 synchronization (sync) events.
The UE shall monitor the downlink link quality based on the cell-specific reference signal in order to detect the downlink radio link quality of the PCell and PSCell. The UE shall estimate the downlink radio link quality and compare it to the thresholds Qout and Qin for the purpose of monitoring downlink radio link quality of the PCell and PSCell. The CRS signals have a constant transmit power and hence provide the UE with a reliable input for its sync evaluation algorithm.